Security documents that must be verifiable on their authenticity are e.g. all kinds of identification documents such as passports, visas, identity cards, driver licenses, bank cards, credit cards, security entrance cards, and further value-documents such as banknotes, shares, bonds, certificates, cheques, lottery tickets and all kinds of entrance tickets such as airplane tickets and railroad season-tickets.
Nowadays, by the availability of markedly improved black-and-white and color copiers it becomes more and more easy to copy documents at high quality hardly to distinguish from the originals.
To protect the above mentioned documents against fraudulent alterations and reproduction by photo-copying different techniques are used such as the melt-laminating or glueing thereto of preprinted plastic overlayers; the printing with special inks; the application of coatings or layers for example loaded with magnetic or fluorescent pigments; coloring or metallizing the substrate of the documents: incorporating holograms; applying fine line printing, watermarks, fibers, security threads, light diffraction marks, liquid crystal marks and/or substances called nacreous, iridiscent or interference pigments.
In a particular case disclosed in U.S. Pat. No. 4,151,666 light-transmissive pigments serving as diffuse reflectors are applied by printing to form a verification pattern in a laminated identification card (I.D. card). In the specification of the same US-P the use of nacreous pigments in verification patterns has been described. Nacreous pigments, also called pearlescent pigments have light-reflection characteristics that change as a function of the viewing or copying angle. The effect of changing color with viewing angle makes that nacreous pigments represent a simple and convenient matter to built in a verification feature associated with a non-copyable optical property.
Interference pigments are in the form of light-reflecting crystal platelets of appropriate thickness to produce color by interference. These pigments exhibit a color play that verges on iridiscence and under a given angle of reflection will allow only the copying of a single color. Whereas other colors appear under different angles of reflection, in other words these pigments show another color to the human eye depending on the observation angle. High nacreous luster is accompanied by high specular reflectance. In most light interference pigments the transmission color is generally the complement of the reflection color.
Observed in transmission, some particular light interference pigments having more pronounced covering aspect show a particular greyish color while observed in reflection mode they have a more pronounced color-shift effect due to their specific built up and composition.
Transmission color of light interference pigments is much weaker than reflection color, which color seen in the reflection mode is called hereinafter "normal" color. [ref. Number 2 in a series of Mearl Technical Bulletins "Nacreous (Pearlescent) Pigments and Interference Pigments by L.M. Greenstein Henry L. Mattin Laboratories Reprinted from Pigment Handbook, Vol. I, Properties and Economics, 2nd Edition, Edited by Peter A. Lewis, (1988) by permission of John Wiley & Sons, Inc. The Mearl Corporation, 41 East 42nd Street. New York, N.Y. 100017, p. 5 and 6].
In interference, the reflection and transmission colors vary with angle of incidence. The reflection maximum and minimum shift to lower wavelengths as the angle of incidence increases (ref. the above mentioned Mearl Technical Bulletin, p. 8. Variation in color with angle of incidence and observation is referred to as geometric metamerism or goniochromatism [ref. Johnston, R. M. Color Eng., 5(3), 42-47, 54 (1967) and Hemmendinger, H. and Johnston R. M. "A Goniospectrophotometer for Color Measurements" in Color 69 (M. Richter. ed). Musterschmidt, Gottingen, Germany (1970)].